Plugin Load/registry information

Hi, I am new to fluentd and want to know more on how external plugins are loaded during fluentd runtime. I bumped into this video: RubyKaigi 2014 | Fluentd Hacking Guide (Fluentd ソースコード完全解説) which (I think) has all the information I need, but the recorded session is in Japanese.

Is there any documentation which has some information on deep dive on the fluentd architecture on the the plugins framework?

Thanks! ET